Remove unnecessary allocations from the layout loop.

src/Avalonia.Layout/LayoutManager.cs

@@ -15,9 +15,15 @@ namespace Avalonia.Layout
     {
         private readonly LayoutQueue<ILayoutable> _toMeasure = new LayoutQueue<ILayoutable>(v => !v.IsMeasureValid);
         private readonly LayoutQueue<ILayoutable> _toArrange = new LayoutQueue<ILayoutable>(v => !v.IsArrangeValid);
+        private readonly Action _executeLayoutPass;
         private bool _queued;
         private bool _running;
 
+        public LayoutManager()
+        {
+            _executeLayoutPass = QueueLayoutPass;
+        }
+
         /// <inheritdoc/>
         public void InvalidateMeasure(ILayoutable control)
         {
@@ -215,7 +221,7 @@ namespace Avalonia.Layout
         {
             if (!_queued && !_running)
             {
-                Dispatcher.UIThread.Post(ExecuteLayoutPass, DispatcherPriority.Layout);
+                Dispatcher.UIThread.Post(_executeLayoutPass, DispatcherPriority.Layout);
                 _queued = true;
             }
         }

src/Avalonia.Layout/LayoutQueue.cs

@@ -1,7 +1,6 @@
 using System;
 using System.Collections;
 using System.Collections.Generic;
-using System.Linq;
 
 namespace Avalonia.Layout
 {
@@ -18,9 +17,11 @@ namespace Avalonia.Layout
             _shouldEnqueue = shouldEnqueue;
         }
 
-        private Func<T, bool> _shouldEnqueue;
-        private Queue<T> _inner = new Queue<T>();
-        private Dictionary<T, Info> _loopQueueInfo = new Dictionary<T, Info>();
+        private readonly Func<T, bool> _shouldEnqueue;
+        private readonly Queue<T> _inner = new Queue<T>();
+        private readonly Dictionary<T, Info> _loopQueueInfo = new Dictionary<T, Info>();
+        private readonly List<KeyValuePair<T, Info>> _notFinalizedBuffer = new List<KeyValuePair<T, Info>>();
+
         private int _maxEnqueueCountPerLoop = 1;
 
         public int Count => _inner.Count;
@@ -60,13 +61,19 @@ namespace Avalonia.Layout
 
         public void EndLoop()
         {
-            var notfinalized = _loopQueueInfo.Where(v => v.Value.Count >= _maxEnqueueCountPerLoop).ToArray();
+            foreach (KeyValuePair<T, Info> info in _loopQueueInfo)
+            {
+                if (info.Value.Count >= _maxEnqueueCountPerLoop)
+                {
+                    _notFinalizedBuffer.Add(info);
+                }
+            }
 
             _loopQueueInfo.Clear();
 
-            //prevent layout cycle but add to next layout the non arranged/measured items that might have caused cycle
-            //one more time as a final attempt
-            foreach (var item in notfinalized)
+            // Prevent layout cycle but add to next layout the non arranged/measured items that might have caused cycle
+            // one more time as a final attempt.
+            foreach (var item in _notFinalizedBuffer)
             {
                 if (_shouldEnqueue(item.Key))
                 {
@@ -74,6 +81,8 @@ namespace Avalonia.Layout
                     _inner.Enqueue(item.Key);
                 }
             }
+
+            _notFinalizedBuffer.Clear();
         }
     }
 }

src/Avalonia.Layout/Layoutable.cs

@@ -562,11 +562,18 @@ namespace Avalonia.Layout
             double width = 0;
             double height = 0;
 
-            foreach (ILayoutable child in this.GetVisualChildren())
+            var visualCount = VisualChildren.Count;
+
+            for (var i = 0; i < visualCount; i++)
             {
-                child.Measure(availableSize);
-                width = Math.Max(width, child.DesiredSize.Width);
-                height = Math.Max(height, child.DesiredSize.Height);
+                IVisual visual = VisualChildren[i];
+
+                if (visual is ILayoutable layoutable)
+                {
+                    layoutable.Measure(availableSize);
+                    width = Math.Max(width, layoutable.DesiredSize.Width);
+                    height = Math.Max(height, layoutable.DesiredSize.Height);
+                }
             }
 
             return new Size(width, height);
@@ -658,9 +665,18 @@ namespace Avalonia.Layout
         /// <returns>The actual size used.</returns>
         protected virtual Size ArrangeOverride(Size finalSize)
         {
-            foreach (ILayoutable child in this.GetVisualChildren().OfType<ILayoutable>())
+            var arrangeRect = new Rect(finalSize);
+
+            var visualCount = VisualChildren.Count;
+
+            for (var i = 0; i < visualCount; i++)
             {
-                child.Arrange(new Rect(finalSize));
+                IVisual visual = VisualChildren[i];
+
+                if (visual is ILayoutable layoutable)
+                {
+                    layoutable.Arrange(arrangeRect);
+                }
             }
 
             return finalSize;